Polycarbonate-based oriented film and retardation film

ABSTRACT

A uniaxially or biaxially oriented film which is made from a specific polycarbonate having a fluorene ring and a glass transition temperature of 165° C. or higher and which has a heat shrinkage factor when it is heated at 90° C. for 500 hours of 0.1% or less, and a ratio of retardation R(450) within the film plane at a wavelength of 450 nm to retardation R(550) within the film plane at a wavelength of 550 nm of 1 to 1.06. This film can be used in a high-quality liquid crystal display device such as a vertical alignment liquid crystal display device and is useful as a retardation film which almost solves a frame problem.

TECHNICAL FIELD

The present invention relates to a polycarbonate-based oriented film anda retardation film. More specifically, it relates to apolycarbonate-based oriented film which is suitably used as aretardation film for liquid crystal display devices and a retardationfilm as one of its uses.

BACKGROUND ART

A retardation film is used in an STN (Super Twisted Nematic) liquidcrystal display device or the like to solve problems such as colorcompensation and the expansion of viewing angle. As the material of aretardation film for color compensation has been used a polycarbonate,polyvinyl alcohol, polysulfone, polyether sulfone or amorphouspolyolefin. Liquid crystalline polymer and discotic liquid crystals havealso been used as the material of a retardation film for the expansionof viewing angle in addition to the above materials.

A vertical alignment liquid crystal display device in which liquidcrystals are aligned almost vertically to a substrate when voltage isoff has already been used in monitors and TVs due to its high contrastand wide viewing angle. It is described in the 1997 Society forinformation display international symposium digest of technical papersat pages 845 to 848 that the use of a retardation film is important toobtain a wide viewing angle.

A retardation film made from a polycarbonate homopolymer produced frombisphenol A as a starting material has been widely used in the above STNliquid crystal display device.

However, as especially a vertical alignment liquid crystal displaydevice has higher quality than an STN liquid crystal display device, ithas been found that a retardation film made from a polycarbonatematerial which has been used in the conventional STN liquid crystaldisplay device cannot obtain sufficiently high display quality. That is,the retardation value and the optical axis of a retardation film arechanged by stress in the step of joining together a retardation filmmade from a polycarbonate homopolymer and a polarizer film, stress inthe step of joining the laminated polarizer film obtained in the abovestep to a liquid crystal display device, or the shrinkage stress of apolarizer film which is produced during a durability test at a hightemperature or at a high temperature and a high humidity, with theresult that the brightness of the screen of the liquid crystal displaydevice becomes nonuniform particularly when black is displayed on theentire screen, thereby deteriorating display quality. The place wherethis brightness nonuniformity appears which depends on the mode of theliquid crystal display device is around the edges of the four sides ofthe screen of the liquid crystal display device in most cases.Therefore, this phenomenon will be referred to as “frame phenomenon” andthis problem will be referred to as “frame problem” in this descriptionhereinafter.

Cellulose acetate, polyolefin and polycarbonate are known as thematerial of the retardation film.

However, a retardation film made from cellulose acetate has poorstability of molecular orientation as cellulose acetate has a high waterabsorption coefficient, thereby making it difficult to use it when ahigh degree of orientation is required within the plane and to suppressvariations in anisotropy within the plane for the same reason. Since apolyolefin having a cyclic skeleton such as a norbornene skeleton has alow photoelastic constant and low intrinsic birefringence, it must bestretched at a high draw ratio to obtain a retardation required for aretardation film. Since a bulky molecular structure such as a norborneneskeleton is employed to obtain a high glass transition point, aretardation film made from the polyolefin has low impact resistance,handling ease and stretchability, easily breaks and often ruptures.Therefore, this film has a lot of problems to be solved when it isproduced or used as a retardation film.

Meanwhile, a polycarbonate comprising an aromatic dihydroxy compound(bisphenol) having two aromatic rings through a bond group out ofaromatic polycarbonates has appropriate flexibility and a high glasstransition point. However, a homopolymer having a bisphenol A skeletonwhich is widely used in an STN mode has no problem with handling easeand stretchability but does have the above frame problem. Therefore, itis difficult to use it in a vertical alignment liquid crystal displaydevice which must have high quality.

There are many kinds of polycarbonates and there are a large number ofexamples in which the polycarbonates are used as retardation films. JP-A7-246661 and JP-A 6-82624 (the term “JP-A” as used herein means an“unexamined published Japanese patent application”) propose retardationfilms made from a polycarbonate comprising a dihydroxy component otherthan a bisphenol A skeleton.

Polycarbonates are divided into aliphatic and aromatic polycarbonates.In general, aliphatic polycarbonates are not used as the material of aretardation film because they have a low glass transition temperatureand poor productivity though they have a low photoelastic constant. Oneof the causes of the frame phenomenon is that stress generated by theshrinkage of a polarizer spreads to a retardation film through anadhesive layer to change the retardation of the retardation film.Therefore, a retardation film having a lower photoelastic constant isconsidered to be preferred because a change in retardation caused bystress becomes smaller, which is not a necessary and sufficientcondition. Meanwhile, aromatic polycarbonates have high production easeand their glass transition temperatures can be easily raised by theexistence of an aromatic ring. As described above, they are actuallyused as the material of a retardation film but they have a problem thattheir photoelastic constants are relatively high. Attempts have alreadybeen made to reduce the photoelastic constant of an aromaticpolycarbonate film, and some homopolymers and copolymers are proposed.

However, in the case of these aromatic polycarbonates, though the reasonis not clear, probably due to the existence of an aromatic ring, it isdifficult to reduce the photoelastic constants of the aromaticpolycarbonates to the level of a commercially available optical filmmade from a polyolefin having a bulky functional group such as the abovenorbornene skeleton. That is, although polycarbonates are superior tothe above polyolefin in handling ease and moldability, it is difficultto reduce their photoelastic constants while realizing a high glasstransition temperature.

JP-A 6-25398 discloses a polycarbonate resin having a high refractiveindex and low birefringence which comprises a structural unitrepresented by the following formula (a):

wherein R₁ to R₄ are each a hydrogen atom, halogen atom, phenyl group oralkyl group having 1 to 3 carbon atoms,and a structural unit represented by the following formula (b):

wherein w is a single bond, alkylidene group, cycloalkylidene group,phenyl substituted alkylidene group, sulfone group, sulfide group oroxide group, R₅ and R₆ are each a hydrogen atom, halogen atom, phenylgroup or alkyl group having 1 to 3 carbon atoms, and m and n are each aninteger of 1 to 4, and which contains the structural unit (b) in anamount of 41 to 95 mol %. It is disclosed in Examples of the abovepublication that polycarbonates (powders) produced by the solutionpolymerization of 9,9-bis(4-hydroxyphenyl)fluorene and bisphenol A inmolar ratios of 85/15 (Example 1), 75/25 (Example 2) and 50/50 (Example3) are dissolved in methylene chloride to obtain films. However, thepublication is silent about uniaxially oriented or biaxially orientedfilms made from the above polycarbonates and therefore about retardationfilms composed of these films as well.

JP-A 2001-318232 discloses an optical film which is made from apolycarbonate containing 1 mol % or more of a recurring unit representedby the following formula (c):

and having a glass transition temperature of 160° C. or higher, andwhich has a heat shrinkage factor when heated at 80° C. for 500 hours of0.07% or less, a hardness measured by a super microhardness meter of 16kg/mm² or more, a thickness of 10 to 200 μm and a retardation (R(550))at a wavelength of 550 nm satisfying |R(550)|≦20 nm. It is disclosed inExample 7 of the above publication that a polycarbonate copolymerproduced by the solution polymerization of 30 mol % of a bisphenolcompound represented by the following formula:

and 70 mol % of bisphenol A is dissolved in methylene chloride to obtaina cast film which is then stretched uniaxially to 1.5 times at 196° C.to obtain an optical film having an R(550) of 5.0 nm.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide apolycarbonate-based oriented film which is suitably used as aretardation film for liquid crystal display devices.

It is another object of the present invention to provide apolycarbonate-based oriented film which is suitably used as aretardation film having various optical anisotropies required forvertical alignment liquid crystal display devices.

It is still another object of the present invention to provide aretardation film which can almost solve the frame problem and canprovide excellent viewing angle characteristics when it is used in alarge-sized liquid crystal display device having a display area of 15inches or more, particularly a large-sized vertical alignment liquidcrystal display device which makes it more difficult to solve the aboveframe problem due to the large area of its polarizer film.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a uni- or bi-axiallyoriented film

-   (A) which comprises a polymer or polymer mixture containing a    recurring unit represented by the following formula (I):    wherein R¹ to R⁸ are each independently a member selected from the    group consisting of hydrogen atom, halogen atom, hydrocarbon group    having 1 to 6 carbon atoms and hydrocarbon-O-group having 1 to 6    carbon atoms, and X is a group represented by the following formula    (I)-1:    wherein R³⁰ and R³¹ are each independently a halogen atom or alkyl    group having 1 to 3 carbon atoms, and n and m are each independently    an integer of 0 to 4,    the polymer and the polymer mixture containing the recurring unit    represented by the above formula (I) in an amount of 30 to 60 mol %    based on the total of all the recurring units of the polymer or    polymer mixture and having a glass transition temperature of 165° C.    or higher,-   (B) which has a heat shrinkage factor when it is heated at 90° C.    for 500 hours under no load of 0.1% or less, and-   (C) which satisfies the following expression (1):    1≦R(450)/R(550)≦1.06  (1)    wherein R(450) and R(550) are retardations within the film plane at    wavelengths of 450 nm and 550 nm, respectively.

According to the present invention, secondly, the above objects andadvantages of the present invention are attained by a retardation filmwhich is the above uniaxially or biaxially oriented film of the presentinvention.

That is, the inventors of the present invention have conducted intensivestudies, paying attention to the molecular structure of a polycarbonatematerial and the physical properties of a retardation film in order touse a polycarbonate which is superior to a cyclopolyolefin having abulky functional group such as the above norbornene skeleton in handlingease and stretchability in a retardation film for vertical alignmentliquid crystal display devices and have found that control factors otherthan the photoelastic constant of the retardation film are important asone of the causes of the above frame problem. When they have conductedfurther studies, they have found that the frame phenomenon can bereduced by controlling some of the factors such as the molecularstructure, glass transition temperature and heat shrinkage factor of apolycarbonate in use even though its photoelastic constant is high andthat the polycarbonate can be used in a retardation film for verticalalignment liquid crystal display devices satisfactorily. The presentinvention has been accomplished based on these findings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an example of a verticalalignment liquid crystal display device comprising the oriented film ofthe present invention as a retardation film;

FIG. 2 is a schematic sectional view of another example of a verticalalignment liquid crystal display device comprising the oriented film ofthe present invention as a retardation film; and

FIG. 3 is a schematic sectional view of still another example of avertical alignment liquid crystal display device comprising the orientedfilm of the present invention as a retardation film.

BEST MODE FOR CARRYING OUT THE INVENTION

A polymer which is the material of a retardation film used in thepresent invention is a specific polycarbonate having a fluorene ring.That is, the polymer is a polycarbonate which comprises a recurring unitrepresented by the following formula (I) in an amount of 30 to 60 mol %,preferably more than 30 mol % and 60 mol % or less based on the total ofall the recurring units constituting the polycarbonate:

In the above formula (I), R¹ to R⁸ are each independently at least onemember selected from the group consisting of hydrogen atom, halogenatom, hydrocarbon group having 1 to 6 carbon atoms and hydrogen-O-grouphaving 1 to 6 carbon atoms. Examples of the hydrocarbon group includealkyl groups such as methyl group and ethyl group, and aryl groups suchas phenyl group. A polycarbonate of the formula (I) in which one of R¹and R³ is a hydrogen atom, the other is a methyl group, one of R⁶ and R⁸is a hydrogen atom and the other is a methyl group is excellent inhandling ease and the like.

X is a group (fluorene component) represented by the following formula.

R³⁰ and R³¹ are each independently a halogen atom or alkyl group having1 to 3 carbon atoms such as methyl group. n and m are each an integer of0 to 4.

A preferred polycarbonate material comprises a recurring unitrepresented by the above formula (I) and a recurring unit represented bythe following formula (II), and the recurring unit represented by theabove formula (I) is contained in an amount of 35 to 60 mol % based onthe total of the recurring units (I) and (II):

In the above formula (II), R⁹ to R¹⁶ are each independently at least onemember selected from the group consisting of hydrogen atom, halogen atomand hydrocarbon group having 1 to 22 carbon atoms, and Y is at least onemember selected from the group consisting of

R¹⁷ to R¹⁹, R²¹ and R²² in Y are each independently a hydrogen atom,halogen atom, alkyl group or hydrocarbon group having 1 to 22 carbonatoms such as aryl group, R²⁰ and R²³ are each independently an alkylgroup or hydrocarbon group having 1 to 20 carbon atoms such as arylgroup, and AR¹ to AR³ are each independently an aryl group having 6 to10 carbon atoms such as phenyl group.

More preferably, the above polycarbonate is a polycarbonate whichcomprises a recurring unit represented by the following formula (III) inan amount of 45 to 55 mol % based on the total of all the recurringunits and a recurring unit represented by the following formula (IV) inan amount of 55 to 45 mol % based on the total of all the recurringunits.

In the above formula (III), R²⁴ and R²⁵ are each independently ahydrogen atom or methyl group. Preferably, R²⁴ and R²⁵ are both methylgroups.

In the above formula (IV), R²⁶ and R²⁷ are each independently a hydrogenatom or methyl group. Preferably, R²⁶ and R²⁷ are both hydrogen atoms.

The above polycarbonate may be a copolymer or a polymer mixture (blendor blend polymer). It may be a mixture of two or more copolymers, amixture of two or more homopolymers, or a mixture of a homopolymer and acopolymer.

When the amount of the recurring unit of the above formula (I) is largerthan 60 mol %, it may be difficult to satisfy the following expression(1) which shows the above retardation wavelength dispersioncharacteristics. When the amount is smaller than 30 mol %, it isdifficult not only to satisfy the above expression (1) but also toobtain a glass transition temperature of 165° C. or higher. To solve theframe problem in the present invention, the glass transition temperaturemust be 165° C. or higher, preferably 200° C. or higher. Thephotoelastic constant is preferably low in order to solve the frameproblem. When the glass transition temperature is low, the frame problemmay occur. For example, even when the photoelastic constant at roomtemperature is as low as about 10×10⁻⁸ cm²/N or less, the frame problemmay occur. Meanwhile, the film of the present invention may have aphotoelastic constant at room temperature of about 30×10⁻⁸ cm²/N or morebut still it can suppress the frame phenomenon. Even when the film has arecurring unit of the above formula (I), if it does not have a glasstransition temperature of 165° C. or higher, its frame phenomenon maybecome problematic.

The above molar ratio can be obtained from the whole bulk of thepolycarbonate constituting the polymer with a nuclear magnetic resonance(NMR) device, for example, whether it is a copolymer or blend polymer.

The above copolymer and/or blend polymer can be produced by a knownmethod. The polycarbonate is advantageously produced by the meltpolycondensation or solid-phase polycondensation of a dihydroxy compoundand phosgene. In the case of a blend, a compatible blend is preferredbut when components are not completely compatible with each other, ifrefractive indices of the components are made the same, opticalscattering between the components can be suppressed and therebytransparency can be improved.

The reason why a glass transition temperature of 165° C. or higher,preferably 200° C. or higher is one of the important factors forattaining the object of the present invention, that is, the suppressionof the frame phenomenon is not fully known. All the causes of the framephenomenon are also not fully known. However, at least the developmentof optical anisotropy of a retardation film by stress is considered asone of the causes and to be connected with the movement of the molecularchain of a polymer material forming the retardation film. Since thetemperature at which a liquid crystal display device is used and thetemperature applied to a retardation film on a liquid crystal displaydevice in the production process are generally about room temperature±50° C., it is considered that as the difference between the device usetemperature/the process temperature and the glass transition temperatureincreases, molecular movement at around room temperature lowers and theframe phenomenon can be reduced more. The term “molecular movement”includes macroscopic molecular movement such as the creep phenomenon ofpolymer.

As one of the causes of the frame phenomenon is connected with themolecular movement of the above polycarbonate material for forming aretardation film as described above, the molecular weight of thematerial is preferably within a certain range. The intrinsic viscosityindicative of molecular weight is preferably 0.4 to 1.1 dl/g, morepreferably 0.5 to 1.0 dl/g. Although the intrinsic viscosity ispreferably higher from the viewpoint of the molecular movement ofpolymer which causes the frame phenomenon, if it is too high, thereoccurs a problem with the moldability of a film, or mass productivity isreduced by a rise in viscosity in the step of polymerizing a polymer.Therefore, it is preferred to maintain the intrinsic viscosity at theabove range.

The oriented film of the present invention may be a uniaxially orientedfilm or a biaxially oriented film.

The oriented film of the present invention can be produced by stretchingan unstretched film. To produce the unstretched film, a known meltextrusion method or solution casting method is used but a solutioncasting method is preferred from the viewpoints of film thicknessnonuniformity and appearance. The solvent used in the solution castingmethod is preferably methylene chloride or dioxolane.

Uniaxial orientation may be either longitudinal orientation ortransverse orientation, or either uniaxial orientation with a free widthor uniaxial orientation with a fixed width. Biaxial orientation may besequential biaxial orientation or simultaneous biaxial orientation.Sequential biaxial orientation may be carried out by stretching in atransverse direction after stretching in a longitudinal direction, orvice versa.

To improve stretchability, the film may contain a known plasticizerexemplified by phthalates such as dimethyl phthalate, diethyl phthalateand dibutyl phthalate, phosphates such as tributyl phosphate, aliphaticdibasic esters, glycerin derivatives and glycol derivatives. At the timeof stretching, the organic solvent used to produce a film may remain inthe film. The amount of the organic solvent is preferably 1 to 20 wt %based on the material of the oriented film.

The oriented film of the present invention must have a heat shrinkagefactor of 0.1% or less when it is heated at 90° C. for 500 hours. Thereason for this is considered to be as follows. Since heat shrinkage isconsidered to be the result of long-time molecular movement as describedabove, it is assumed that the retardation of a film having a large heatshrinkage factor changes for a long time even when it is alone. Thedimensional change of the film itself produces stress between it andglass or other optical film in contact therewith through an adhesivelayer with the result that a retardation change is induced and leads tothe occurrence of the frame phenomenon. When the inventors of thepresent invention have conducted intensive studies, they have found thatthe frame phenomenon can be further suppressed when the heat shrinkagefactor of the film when it is heated at 90° C. for 500 hours is 0.1% orless in addition to the above other factors of suppressing the framephenomenon. It is presumed that when the temperature of the actual useenvironment to be applied to the retardation film is considered to beabout 80° C. at maximum in consideration of heat from a backlight in aliquid crystal display device such as a liquid crystal TV or liquidcrystal monitor, there will be no problem. That is, the temperature 90°C. is set by adding a margin of 10° C. to 80° C. which is presumed to bethe highest temperature of the actual use environment. The heatshrinkage factor of a retardation film differs according to a directionfor measuring it within the film plane. A specific measuring method willbe described in Examples and limited by a heat shrinkage factor in aslow axis direction having the largest refractive index within the filmplane. The heat shrinkage factor is preferably 0.08% or less.

An oriented film made from an amorphous polymer such as a polycarbonatetends to have larger heat shrinkage than an unoriented film made fromthe same polymer. To reduce heat shrinkage, for example, a material andproduction method must be worked out. It should be noted that theabove-described specific polycarbonate is a material having small heatshrinkage and excellent dimensional stability when it is heated after itis stretched.

The oriented film of the present invention must further satisfy thefollowing expression (1):1≦R(450)/R(550)≦1.06  (1)wherein R(450) and R(550) are retardations within the film plane atwavelengths of 450 nm and 550 nm, respectively.

In a vertical alignment mode liquid crystal display device, since liquidcrystals are aligned substantially vertically at the time of displayingblack when voltage is off, to obtain a good viewing angle by opticallycompensating this, the oriented film of the present invention preferablysatisfies the following expressions (2) and (3) when it is a uniaxiallyoriented film:R(550)>K(550)  (2)R(550)>20 nm  (3)wherein R(550) is as defined in the above expression (1) and K(550) is avalue (nm) defined by the following expression (4) at a wavelength of550 nm:K=[(n _(x) +n _(y))/2−n _(z) ]×d  (4)wherein n_(x), n_(y) and n_(z) are refractive indices in x, y and z axisdirections of the film, respectively, and d is the thickness (nm) of thefilm.

The biaxially oriented film of the present invention preferablysatisfies the following expression (2′) and the above expression (3):R(550)≦K(550)  (2′)wherein R(550) and K(550) are as defined in the above expressions,or the above expression (2′) and the following expressions (3′) and (5)when it is a biaxially oriented film:R(550)≦20 nm  (3′)K(550)≧50 nm  (5)wherein R(550) and K(550) are as defined in the above expressions.

The biaxially oriented film which satisfies the above expressions (2′)and (3) more preferably satisfies the following expression (1′) in placeof the above expression (1):1≦R(450)/R(550)≦1.05  (1′)wherein R(450) and R(550) are as defined in the above expressions.

The oriented film of the present invention which satisfies the abovecharacteristic properties can carry out mainly the optical compensationof a liquid crystal cell layer and a polarizer in a vertical alignmentmode as a retardation film.

Further, the uniaxially oriented film of the present invention havingthe above characteristic properties can carry out the opticalcompensation of a vertical alignment mode liquid crystal display devicewell when at least one of the film is combined with at least one otherretardation film which satisfies the following expressions (6), (7), (8)and (9) at the same time, preferably one of the film is combined withone other retardation film. In this case, the uniaxially oriented filmof the present invention carries out mainly the compensation of theviewing angle of a polarizer film for a vertical alignment mode liquidcrystal display device whereas the other retardation film whichsatisfies the following expressions (6), (7), (8) and (9) at the sametime carries out mainly the optical compensation of a liquid crystalcell.1≦R(450)/R(550)≦1.06  (6)R(550)≦K(550)  (7)R(550)≦20 nm  (8)K(550)≧50 nm  (9)wherein R(450), R(550) and K(550) are as defined in the aboveexpressions.

It is particularly preferred to use the biaxially oriented film of thepresent invention having the above characteristic properties inconjunction with other uniaxially anisotropic retardation film whichsatisfies the following expression (10):R(550)=2×K(550)  (10)in a vertical alignment mode in order to expand the viewing angle of aliquid crystal display device.

Wavelength dispersion characteristics are also important from theviewpoint of the optical compensation of a vertical alignment modeliquid crystal display device. The oriented film of the presentinvention must satisfy the above expression (1), preferably thefollowing expression (1″) from the viewpoints of matching with thewavelength dispersion of liquid crystals and the compensation of theviewing angle of a polarizer film:1.01≦R(450)/R(550)≦1.05  (1″).

Particularly in a vertical alignment mode liquid crystal display devicecomprising a circular polarizer film which generates circularlypolarized light, the above expression (1), preferably the aboveexpression (1″) is satisfied. The term “circular polarizer film” denotesa circular polarizer film in which the polarizing axis of a polarizerfilm and the slow axis of a retardation film are set to 45° or 135°. Itis known that a retardation film used in an STN liquid crystal displaydevice preferably has an R(450)/R(550) larger than a value specified bythe above expression (1) (more than 1.06) but the above expression (1)is preferably satisfied in a vertical alignment mode liquid crystaldisplay device.

Although the uniaxially oriented film and the biaxially oriented film ofthe present invention can be produced by uniaxial orientation andbiaxial orientation, respectively, as described above, it has been foundthat a heat shrinkage factor of 0.1% or less can be effectively attainedby heating after stretching. As for the heating conditions afterstretching, the temperature is preferably in the range of (the glasstransition temperature of the oriented film −50° C.) to (the glasstransition temperature +30° C.). The retardation film must have a largeretardation value. In general, an alignment structure formed byorientation is relaxed by heating to reduce the retardation value inmost cases. According to the present invention, to suppress this, thedraw ratio is preferably not changed or slightly reduced. Stated morespecifically, a reduction in the draw ratio is 0 to 30%, more preferably1 to 20% of the draw ratio just before reduction. The heating time whichdepends on the heating temperature is preferably 1 to 200 seconds. Theheating for the above orientation includes heating at the end of thestretching step by reducing the draw ratio when the orientation iscontinuous transverse orientation.

To effectively suppress the frame phenomenon, the change of R(550) after500 hours of heating at 90° C. is preferably ±3 nm or less. It is morepreferably ±2 nm or less. For the evaluation of this, there are changesin the physical properties of the oriented film alone. When this valueis large, the frame phenomenon may occur.

As described above, the unstretched film for obtaining the oriented filmof the present invention is preferably produced by the solution castingmethod. In this case, to suppress heat shrinkage, the residue of thesolvent in the oriented film is preferably 0.9 wt % or less, morepreferably 0.7 wt % or less.

The water absorption coefficient of the polymer material of the orientedfilm is preferably 1 wt % or less, more preferably 0.8 wt % or less,particularly preferably 0.5 wt % or less. When a polymer having a highwater absorption coefficient is used, the frame phenomenon may be seenmarkedly in a moist heat test.

From the viewpoint of productivity, the oriented film of the presentinvention as a retardation film can be preferably rolled in which itsslow axis within the film plane is existent in a direction parallel tothe width direction of the film. In a vertical alignment mode liquidcrystal display device, a polarizer film and a retardation film may bejoined together with an adhesive layer therebetween in such a mannerthat the transmission axis of the polarizer film becomes perpendicularor parallel to the slow axis of the retardation film. Since a polarizerfilm comprising iodine which is widely and generally used is produced bycontinuous longitudinal uniaxial orientation, its transmission axis isgenerally existent in a direction perpendicular to the flow direction ofa roll. Therefore, in the above vertical alignment mode liquid crystaldisplay device, when the polarizer film and the retardation film areused in such a manner that the transmission axis of the polarizer filmand the slow axis of the retardation film become parallel to each other,if a laminated polarizer film can be produced by joining together thepolarizer film and the retardation film with an adhesive layertherebetween by a roll-to-roll system, productivity will be greatlyimproved. To realize this, the slow axis of the rolled retardation filmmust be existent in the width direction of the film.

The oriented film of the present invention is preferably transparentwith a haze value of 3% or less, preferably 1% or less and a total lighttransmittance of 85% or more, preferably 90% or more.

The oriented film may further contain an ultraviolet light absorber suchas phenylsalycylic acid, 2-hydroxybenzophenone or triphenyl phosphate,bluing agent for changing color, antioxidant and the like.

The thickness of the oriented film of the present invention is notlimited but preferably 1 to 400 μm. The oriented film and theretardation film of the present invention include what are called“sheet” or “plate”.

It is known that a retardation film gives a different retardation valueto obliquely input light from straight input light. The 3-D refractiveindices of a polymer material are represented by n_(x), n_(y) and n_(z)which are defined as follows.

-   n_(x): refractive index in the main stretching direction within the    plane of the retardation film-   n_(y): refractive index in a direction perpendicular to the main    stretching direction within the plane of the retardation film-   n_(z): refractive index in the normal direction of the surface of    the retardation film.

The expression “main stretching direction” denotes a stretchingdirection in the case of uniaxial orientation and a stretching directionfor improving the degree of orientation in the case of biaxialorientation, or the main orientation direction of the main chain of apolymer in terms of chemical structure. In the present invention, thedirection of the maximum refractive index within the plane is called “nxdirection” (slow axis). In the present invention, the retardation valueR is represented by R=(n_(x)−n_(y))×d (nm) (d is the thickness (nm) ofthe film).

The 3-D refractive indices are measured by polarization analysis whichis a technique for analyzing the polarization of output light obtainedby inputting polarized light to a retardation film. In the presentinvention, the optical anisotropy of a retardation film is regarded asan index ellipsoid, and the 3-D refractive indices are obtained from aknown index ellipsoid expression. Since the 3-D refractive indices havedependence on the wavelength of a light source in use, they arepreferably defined by the wavelength of the light source in use.

The retardation film of the present invention can carry out the opticalcompensation of all kinds of liquid crystal cells such as bend alignmentcell, vertical alignment cell, in-plane switching mode cell, twistnematic cell and cholesteric mode cell compensated by an opticalcompensation film. Further, it may be used as an optical film for use ina liquid crystal projector.

Particularly, the uniaxially oriented film of the present inventionpreferably satisfies the following expression (11):40 nm≦R(550)≦300 nm  (11),more preferably the following expression (12):50 nm≦R(550)≦200 nm  (12)from the viewpoint of optical compensation as a retardation film for avertical alignment mode liquid crystal display device.

A retardation film which is the uniaxially oriented film of the presentinvention satisfying the following expression (13) is effective from theviewpoint of optical compensation:R(550)=2×K(550)  (13).

As described above, a combination of a retardation film which satisfiesthe above characteristic properties and another retardation film forcarrying out the optical compensation of a liquid crystal cell can carryout mainly the optical compensation of a polarizer film in a verticalalignment mode liquid crystal display device. A retardation film whichsatisfies the above expressions (2), (11) and (12) can compensate anaxial change when light is incident upon the polarizer film obliquely.

The biaxially oriented film of the present invention which satisfies thefollowing expressions (14) and (15):0 nm≦R(550)≦10 nm  (14)60 nm≦K(550)≦400 nm  (15),more preferably the following expressions (16) and (17):0 nm≦R(550)≦5 nm  (16)70 nm≦K(550)≦300 nm  (17)at the same time is effective from the viewpoint of optical compensationas a retardation film for a vertical alignment mode liquid crystaldisplay device.

With reference to the accompanying drawings, examples of a verticalalignment mode liquid crystal display device comprising the orientedfilm of the present invention as a retardation film will be described.

FIG. 1 shows a vertical alignment liquid crystal display devicecomprising a uniaxially oriented film 2 and a biaxially oriented film 4as optical compensation films. In FIG. 1, reference numeral 1 denotes apolarizer, 2 a uniaxially oriented film, 3 a vertically aligned liquidcrystal cell, 4 a biaxially oriented film, 5 a polarizer and 6 a backlight. Although numeral 1 in FIG. 1 denotes the polarizer on anobserver's side, as the uniaxially oriented film substantially serves asa film for compensating the viewing angle of a polarizer, it is situatedpreferably at a position closest to the polarizer 1 or 5, morepreferably at a position closest to the polarizer 1 as shown in FIG. 1.The uniaxially oriented film 2 preferably satisfies the followingexpressions (2) and (3):R(550)>K(550)  (2)R(550)>20 nm  (3),more preferably the above expression (2) and the following expression(11):40 nm≦R(550)≦300 nm  (11),much more preferably the above expression (2) and the followingexpression (12):50 nm≦R(550)≦200 nm  (12),particularly preferably the above expressions (2) and (12) and thefollowing expression (13):R(550)≧2×K(550)  (13).

Since the biaxially oriented film 4 in FIG. 1 mainly functions as anoptical compensation film for a vertically aligned liquid crystal layer,it preferably satisfies the following expression (2′):R(550)≦K(550)  (2′),more preferably the following expressions (3′) and (5):R(550)≦20 nm  (3′)K(550)≧50 nm  (5),much more preferably the following expressions (16) and (17):R(550)≦5 nm  (16)K(550)≧90 nm  (17).

Two or more films may be used as the biaxially oriented film 4 at theposition shown in FIG. 1.

FIG. 2 shows a vertical alignment liquid crystal display devicecomprising two biaxially oriented films. In FIG. 2, reference numeral 7denotes a polarizer, 8 a biaxially oriented film, 9 a vertically alignedliquid crystal cell, 10 a biaxially oriented film, 11 a polarizer and 12a back light. In FIG. 2, numerals 8 and 10 preferably denote filmshaving the same characteristic properties. In this case, thecompensation of the viewing angles of the liquid crystal layer and thepolarizer is carried out with the two films. The biaxially orientedfilms 8 and 10 preferably satisfy the above expression (2′), morepreferably the above expression (2′) and the following expression (3):R(0.550)>20 nm  (3),much more preferably the above expression (2′) and the followingexpressions (18) and (19):20 nm<R(550)≦300 nm  (18)30 nm≦K(550)≦500 nm  (19),particularly preferably the above expression (2′) and the followingexpressions (20) and (21):20 nm<R(550)≦150 nm  (20)30 nm≦K(550)≦300 nm  (21) at the same time.

FIG. 3 shows a vertical alignment liquid crystal display devicecomprising one biaxially oriented film. In FIG. 3, reference numeral 13denotes a polarizer, 14 a vertically aligned liquid crystal cell, 15 abiaxially oriented film, 16 a polarizer and 17 a back light. Thecompensation of the viewing angles of the liquid crystal layer and thepolarizer is carried out with only one film. The preferredcharacteristic properties of the biaxially oriented film are the same asthe above films of FIG. 2.

A plurality of the retardation films of the present invention may beused in a liquid crystal display device, or the retardation film of thepresent invention may be used in combination with another retardationfilm made from a polycarbonate, amorphous polyolefin, polyether sulfone,polycarbonate, polysulfone or cellulose acetate, a substrate coated withpolymer liquid crystals, or an aligned and cured discotic liquid crystallayer. They may be combined in the liquid crystal display device or maybe combined with a polarizer film.

When the retardation film of the present invention which satisfies theseis used in a vertical alignment mode liquid crystal display device,there can be provided a liquid crystal display device having excellentviewing angle characteristics.

The oriented film of the present invention can be advantageously used asa retardation film for not only liquid crystal display devices but alsoemission devices such as organic electroluminescence devices, plasmadisplays, field emission devices and CRT's, electrophoresis devices,optical engines for projectors, optical pick-up devices, image pick-updevices, optical arithmetic devices, optical read/write devices andoptical read/write media.

EXAMPLES

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

Evaluation Methods

The characteristic properties of the material described in this textwere evaluated by the following methods.

(1) Measurement of Retardation Value (R=Δn·d (nm)), K Value

The retardation R value which is the product of birefringence Δn andfilm thickness d and Nz were measured with the M150 (trade name)spectroscopic ellipsometer of Nippon Bunko Co., Ltd. The R value wasmeasured while incident light was perpendicular to the surface of thefilm. The K value (nm) was obtained by measuring a retardation value ateach angle by changing the angle between incident light and the surfaceof the film, obtaining 3-D refractive indices n_(x), n_(y) and n_(z) bycurve fitting the obtained values based on a known index ellipsoidexpression, and inserting them into the following equation (4):K=((n _(x) +n _(y))/2−n _(z))*d  (4)wherein n_(x), n_(y), n_(z) and d are as defined hereinabove.(2) Measurement of Water Absorption Coefficient

The water absorption coefficient of the film was measured in accordancewith “water absorption coefficients of plastics and method of testingboiling water absorption coefficient” specified in JISK 7209 except thatthe thickness of the dried film was set to 130±50 μm. The test samplewas 50 mm×50 mm square in size and immersed in water heated at 25° C.for 24 hours to measure its weight change. The unit is %.

(3) Measurement of Glass Transition Temperature (Tg) of Polymer

This was measured with the DSC2920 Modulated DSC of TA Instruments Co.,Ltd. This measurement was made on a flake or chip not after the moldingof a film but after the polymerization of a resin.

(4) Measurement of Film Thickness

This was measured with the electronic micrometer of Anritsu Corporation.

(5) Measurement of Polymer Copolymerization Ratio

This was measured with the JNM-alpha600 proton NMR of JEOL Ltd. In thecase of a copolymer of bisphenol A and biscresol fluorene, heavy benzenewas used as a solvent and the copolymerization ratio was calculated fromthe intensity ratio of the protons of the respective methyl groups.

(6) Measurement of Heat Shrinkage Factor

Three rectangular samples having a length of 150 mm in the slow axisdirection of the film and a width of 10 mm in a direction perpendicularto the above direction were cut out. The three samples were marked withdots for the measurement of heat shrinkage factor in a lengthwisedirection (150 mm) at intervals of 100 mm. They were heated in ahigh-temperature chamber at 90° C. for 500 hours while tension was notapplied thereto, taken out from the chamber at room temperature andcooled for 24 hours to measure the intervals between dots. Thismeasurement was carried out at room temperature (23° C.) with a readingmicroscope. The heat shrinkage factor was obtained from the followingequation (22) and the average value of the three samples was taken asheat shrinkage factor. Heat shrinkage factor (%)=|(size beforetreatment)−(size after treatment)/(size before treatment)|×100 (22).

(7) Measurement of Residual Solvent in Retardation Film

About 5 g of the retardation film was sampled and dried with a hot airdrier at 230° C. for 1 hour to obtain the residue of the solvent in theretardation film from a weight change before and after the treatment.

(8) Measurement of Intrinsic Viscosity of Polymer

The intrinsic viscosity of a polymer was measured in methylene chlorideat 20° C. with an Ubbelohde viscometer.

(9) Observation of Frame Phenomenon

The commercially available HLC2-5618 polarizer film of Sanritsu Co.,Ltd. was used as a polarizer film, and the polarizer film (0° C.),retardation film (0° C.), glass and polarizer film (90° C.) were joinedtogether with an adhesive layer to prepare a test sample. The angleswithin the parentheses indicate the in-plane lamination angle of thetransmission axis of the polarizer film and the in-plane laminationangle of the slow axis of the retardation film. This test sample wasplaced on the back light in such a manner that the retardation film wassituated on an upper side, and a light leak was observed to see a framephenomenon in a dark room. The test sample measured 291 mm×362 mm. Afterlamination, the test sample was heated at 50° C. for 15 minutes underpressure. 24 hours after the test sample was cooled to room temperature,it was observed at a temperature of 23° C. This observation is referredto as “initial evaluation”. 500 hours after the test sample was placedin a high-temperature chamber at 60° C., it was taken out from thechamber and left at room temperature for 24 hours to observe a framephenomenon at room temperature (23° C.). This observation after 500hours of heating is referred to as “evaluation after 500 hours”. Theobservation of the frame phenomenon was carried out at the beginning andafter 500 hours. Since the transmission axis of the polarizer film isperpendicular to the slow axis of the retardation film in the aboveconstitution, if there is no frame phenomenon, a black color isdisplayed on the entire screen but if there is a strong framephenomenon, a light leak occurs at the four corners of the test sample.The brightness of the test sample was measured from a directionperpendicular to the test sample with the LS110 brightness meter ofMinolta Co., Ltd. The measurement points were about 2 cm away from eachcorner and the average value of the four measurement data was taken asfour-corner brightness. One center point was measured and taken ascentral brightness. Further, the frame phenomenon was checked with theeye under an illuminance of about 20 lux after 500 hours. When the framephenomenon was seen, the sample was evaluated as NG and when observationwas difficult, the sample was evaluated as OK.

(10) Change of R(550)

The change of the retardation value R(550) was observed at a measurementwavelength of 550 nm after 500 hours of heating at 90° C. The evaluationresult is represented by |(initial value)−(after 500 hours)| in Table 1.

The monomer structures of the polycarbonates used in the followingExamples and Comparative Examples are shown below.

Example 1

An aqueous solution of sodium hydroxide and ion exchange water were fedto a reactor equipped with a stirrer, thermometer and reflux condenser,monomers [A] and [D] having the above structures were dissolved in thesolution in a molar ratio shown in Table 1, and a small amount ofhydrosulfite was added to the resulting solution. Methylene chloride wasthen added to the solution, and phosgene was blown into it at 20° C. inabout 60 minutes. Further, p-tert-butylphenol was added to emulsify thesolution, and triethylamine was added and stirred at 30° C. for about 3hours to complete a reaction. After the end of the reaction, an organicphase was dispensed, and methylene chloride was evaporated to obtain apolycarbonate copolymer. The composition of the obtained copolymer wasalmost the same as the ratio of the charged monomers shown in Table 1.

This copolymer was dissolved in methylene chloride to prepare a dopehaving a solids content of 18 wt %. A cast film was formed from thisdope to obtain an unstretched film. The residue of the solvent in theunstretched film was 0.9 wt %. After this film was stretched to 1.35times at 220° C. with a transverse uniaxial stretching machine, the drawratio was reduced to 1.33 times at the last part of the uniaxialstretching machine to carry out heat setting at 222° C. for 7 seconds soas to obtain a uniaxially oriented film. The evaluation results of thecharacteristic properties of this film are shown in Table 1. The slowaxis of this retardation film was existent in a direction (mainstretching direction) perpendicular to the flow direction of thetransverse uniaxial stretching machine.

Further, a frame test was made on this retardation film. The results areshown in Table 2. It was found that the frame phenomenon of theretardation film was at an insignificant level as shown in Table 2.

For this frame test, the rolled polarizer film and the rolledretardation film were joined together with an adhesive layer byroll-to-roll in such a manner that the transmission axis (perpendicularto the longitudinal direction) of the polarizer film became parallel tothe slow axis of the retardation film. When the frame test was also madeon this laminate, it was found that the frame phenomenon of the laminatewas at an insignificant level as well.

This uniaxially oriented film was evaluated using the commerciallyavailable VL-151VA liquid crystal monitor making use of a verticalalignment mode manufactured by Fujitsu Limited. This commerciallyavailable liquid crystal display device comprises two retardation filmsand a liquid crystal cell sandwiched between the retardation films. Theretardation film on the front side which was the observer's side of theliquid crystal cell was removed, and the above uniaxially oriented filmwas laminated on the liquid crystal cell instead in such a manner thatthe transmission axis (polarization axis) of the polarizer film and theslow axis of the uniaxially oriented film became parallel to each other.The lamination angle between the polarizer film and the liquid crystalcell was made the same as that of the commercially available product.Further, the retardation film on the rear side of the liquid crystalcell of the commercially available product was removed, the aboveunstretched film was stretched to 1.7 times at 212° C. with alongitudinal uniaxial stretching machine and to 2 times at 220° C. witha transverse uniaxial stretching tenter to obtain a biaxially orientedfilm (R(550)=3.2 nm, K(550)=192.3 nm), and this biaxially oriented filmwas laminated on the liquid crystal cell with an adhesive layertherebetween in such a manner that the transmission axis of thepolarizer film became parallel to the slow axis of the uniaxiallyoriented film. The lamination angle between the polarizer film and theliquid crystal cell was made the same as that of the commerciallyavailable product. When the viewing angle was checked with the eye, itwas found that the viewing angle was wider than that of the commerciallyavailable product and that a color shift by the viewing angle could beconsiderably suppressed.

Example 2

A polycarbonate copolymer was obtained in the same manner as in Example1 except that monomers shown in Table 1 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 1, itwas stretched to 1.4 times at 220° C. with a longitudinal uniaxialstretching machine. The draw ratio was then reduced to 1.39 times at thelast part of the longitudinal uniaxial stretching machine to carry outheat setting at 221° C. for 8 seconds so as to obtain a uniaxiallyoriented film. The evaluation results of the characteristic propertiesof this film are shown in Table 1.

Further, a frame test was made on this uniaxially oriented film. Theresults are shown in Table 2. It was found that the frame phenomenon ofthe uniaxially oriented film was at an insignificant level as shown inTable 2.

Example 3

A polycarbonate copolymer was obtained in the same manner as in Example1 except that monomers shown in Table 1 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 1, itwas stretched to 1.2 times at 233° C. with a longitudinal uniaxialstretching machine. Without reducing the draw ratio at the last part ofthe longitudinal uniaxial stretching machine, heat setting was carriedout at 240° C. for 10 seconds to obtain a uniaxially oriented film. Theevaluation results of the characteristic properties of this film areshown in Table 1.

Further, a frame test was made on this uniaxially oriented film. Theresults are shown in Table 2. It was found that the frame phenomenon ofthe uniaxially oriented film was at an insignificant level as shown inTable 2.

Comparative Example 1

A polycarbonate copolymer was obtained in the same manner as in Example1 except that monomers shown in Table 1 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. A film was formed by changing drying conditions from those ofExample 1, and the residue of the solvent in the unstretched film wasadjusted to 3 wt %. This film was stretched to 1.3 times at 200° C. witha longitudinal uniaxial stretching machine to obtain a uniaxiallyoriented film. The evaluation results of the characteristic propertiesof this film are shown in Table 1.

Further, a frame test was made on this uniaxially oriented film. Theresults are shown in Table 2. The frame phenomenon of this uniaxiallyoriented film was confirmed with the eye as shown in Table 2. It wasfound that a change in brightness was large at the four corners after adurability test and therefore a retardation film of interest could notbe obtained.

Comparative Example 2

A polycarbonate homopolymer was obtained in the same manner as inExample 1 except that a monomer shown in Table 1 was used. Thecomposition of the obtained homopolymer was almost the same as the ratioof the charged monomer. After a film was formed in the same manner as inExample 1, it was stretched to 1.3 times at 156° C. with a longitudinaluniaxial stretching machine. The draw ratio was then reduced to 1.29times at the last part of the longitudinal uniaxial stretching machineto carry out heat setting at 170° C. for 10 seconds so as to obtain auniaxially oriented film. The evaluation results of the characteristicproperties of this film are shown in Table 1.

Further, a frame test was made on this uniaxially oriented film. Theresults are shown in Table 2. The frame phenomenon of this uniaxiallyoriented film was confirmed with the eye as shown in Table 2. It wasfound that the difference in brightness between the center and the fourcorners was large even at the beginning and became larger after thedurability test, and therefore a retardation film of interest could notbe obtained. TABLE 1 Ex. 1 Ex. 2 Ex. 3 C. Ex. 1 C. Ex. 2 Structure ofmonomer 1 [A] [B] [C] [A] [A] (mol % of charged monomer) (50) (55) (53)(50) (100) Structure of monomer 2 [D] [D] [D] [D] (mol % of chargedmonomer) (50) (45) (47) (50) Heat shrinkage factor (%) 0.05 0.06 0.060.20 0.09 Glass transition temperature (° C.) 210 211 230 210 156 R(450)(nm) 112.9 148.2 99.4 115.9 322.5 R(550) (nm) 109.6 145.3 96.5 112.5298.6 R(450)/R(550) 1.03 1.02 1.03 1.03 1.08 K(550) (nm) 54.6 72.7 48.356.2 149.3 Film thickness after stretching (μm) 75 70 67 87 76 Residueof solvent (%) 0.3 0.4 0.4 1.2 0.1 Water absorption coefficient (wt %)0.2 0.2 0.2 0.2 0.2 Intrinsic viscosity (dl/g) 0.78 0.92 0.65 0.78 0.78Change in R(550) 0.6 1.2 1.0 3.6 2.9Ex. = Example,C. Ex. = Comparative Example

TABLE 2 Initial evaluation (cd/m²) Evaluation after 500 hours (cd/m²)Visual Four-corner brightness Central brightness Four-corner brightnessCentral brightness evaluation Ex. 1 0.35 0.33 0.36 0.34 OK Ex. 2 0.370.34 0.38 0.35 OK Ex. 3 0.36 0.34 0.38 0.35 OK C. Ex. 1 0.35 0.32 0.500.33 NG C. Ex. 2 0.46 0.35 0.71 0.37 NGEx. = Example,C. Ex. = Comparative Example

Example 4

An aqueous solution of sodium hydroxide and ion exchange water were fedto a reactor equipped with a stirrer, thermometer and reflux condenser,monomers [A] and [D] having the above structures were dissolved in thesolution in a molar ratio shown in Table 3, and a small amount ofhydrosulfite was added to the resulting solution. Methylene chloride wasthen added to the solution, and phosgene was blown into it at 20° C. inabout 60 minutes. Further, p-tert-butylphenol was added to emulsify thesolution, and triethylamine was added and stirred at 30° C. for about 3hours to complete a reaction. After the end of the reaction, an organicphase was dispensed, and methylene chloride was evaporated to obtain apolycarbonate copolymer. The composition of the obtained copolymer wasalmost the same as the ratio of the charged monomers shown in Table 3.

This copolymer was dissolved in methylene chloride to prepare a dopehaving a solids content of 18 wt %. A cast film was formed from thisdope to obtain an unstretched film. The residue of the solvent in theunstretched film was 0.8 wt %. Further, this film was stretched to 1.4times at 212° C. with a longitudinal uniaxial stretching machine and to2.0 times at 225° C. with a transverse uniaxial stretching tenter. Thedraw ratio was then reduced to 1.95 times at the last part of thetransverse uniaxial stretching machine to carry out heat setting at 225°C. for 10 seconds so as to obtain a biaxially oriented retardation film.The evaluation results of the characteristic properties of this film areshown in Table 3. The slow axis of this biaxially oriented retardationfilm was existent in a direction (main stretching direction)perpendicular to the flow direction of the transverse uniaxialstretching machine.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 4. It was found that the framephenomenon of the biaxially oriented retardation film was at aninsignificant level as shown in Table 4.

For this frame test, the rolled polarizer film and the rolled biaxiallyoriented retardation film were joined together with an adhesive layer byroll-to-roll in such a manner that the transmission axis (perpendicularto the longitudinal direction) of the polarizer film became parallel tothe slow axis of the biaxially oriented retardation film. When the frametest was also made on this laminate, it was found that the framephenomenon of the laminate was at an insignificant level as well.

This biaxially oriented retardation film was evaluated using thecommercially available VL-151VA liquid crystal monitor making use of avertical alignment mode manufactured by Fujitsu Limited. Thiscommercially available liquid crystal display device comprises tworetardation films and a liquid crystal cell sandwiched between theretardation films. The above biaxially oriented retardation film waslaminated in place of these retardation films in such a manner that thetransmission axis of the polarizer film and the slow axis of thebiaxially oriented retardation film became parallel to each other. Thelamination angle of the polarizer film was made the same as that of thecommercially available product. When the viewing angle was checked withthe eye, it was found that the viewing angle was wider than that of thecommercially available product and that a color shift by the viewingangle could be considerably suppressed.

Example 5

A polycarbonate copolymer was obtained in the same manner as in Example4 except that monomers shown in Table 3 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 4, itwas stretched to 1.3 times at 214° C. with a longitudinal uniaxialstretching machine and then to 2.0 times at 227° C. with a transverseuniaxial stretching tenter. The draw ratio was then reduced to 1.95times at the last part of the transverse uniaxial stretching machine tocarry out heat setting at 227° C. for 10 seconds so as to obtain abiaxially oriented retardation film. The evaluation results of thecharacteristic properties of this film are shown in Table 3.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 4. It was found that the framephenomenon of the biaxially oriented retardation film was at aninsignificant level as shown in Table 4.

Example 6

A polycarbonate copolymer was obtained in the same manner as in Example4 except that monomers shown in Table 3 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 4, itwas stretched to 1.3 times at 233° C. with a longitudinal uniaxialstretching machine and then to 2.0 times at 240° C. with a transverseuniaxial stretching tenter. Without reducing the draw ratio at the lastpart of the transverse uniaxial stretching machine, heat setting wascarried out at 245° C. for 10 seconds to obtain a biaxially orientedretardation film. The evaluation results of the characteristicproperties of this film are shown in Table 3.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 4. It was found that the framephenomenon of the biaxially oriented retardation film was at aninsignificant level as shown in Table 4.

Example 7

A polycarbonate copolymer was obtained in the same manner as in Example4 except that monomers shown in Table 3 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 4, itwas stretched to 1.6 times at 169° C. with a longitudinal uniaxialstretching machine and then to 2.2 times at 170° C. with a transverseuniaxial stretching tenter. The draw ratio was then reduced to 2.15times at the last part of the transverse uniaxial stretching machine tocarry out heat setting at 171° C. for 10 seconds so as to obtain abiaxially oriented retardation film. The evaluation results of thecharacteristic properties of this film are shown in Table 3. The slowaxis of this biaxially oriented retardation film was existent in adirection (main stretching direction) perpendicular to the flowdirection of the transverse uniaxial stretching machine.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 4. It was found that the framephenomenon of the biaxially oriented retardation film was at aninsignificant level as shown in Table 4.

For this frame test, the rolled polarizer film and the rolled biaxiallyoriented retardation film were joined together with an adhesive layer insuch a manner that the transmission axis (perpendicular to thelongitudinal direction) of the polarizer film became parallel to theslow axis of the biaxially oriented retardation film. When the frametest was also made on this laminate, it was found that the framephenomenon of the laminate was at an insignificant level as well.

This biaxially oriented retardation film was evaluated using thecommercially available VL-151VA liquid crystal monitor making use of avertical alignment mode manufactured by Fujitsu Limited. Thiscommercially available liquid crystal display device comprises tworetardation films and a liquid crystal cell sandwiched between theretardation films. The above biaxially oriented retardation film waslaminated on only the polarizer on the observer's side in place of theretardation film in such a manner that the transmission axis of thepolarizer film and the slow axis of the biaxially oriented retardationfilm became parallel to each other, and only the polarizer was existenton the rear side. The lamination angle of the polarizer film was madethe same as that of the commercially available product. When the viewingangle was checked with the eye, it was found that the viewing angle waswider than that of the commercially available product and that a colorshift by the viewing angle could be considerably suppressed.

Comparative Example 3

A polycarbonate copolymer was obtained in the same manner as in Example4 except that monomers shown in Table 3 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. A film was formed by changing drying conditions from those ofExample 4, and the residue of the solvent in the unstretched film wasadjusted to 3 wt %. This film was stretched to 1.3 times at 200° C. witha longitudinal uniaxial stretching machine and then to 2.0 times at 210°C. with a transverse uniaxial stretching tenter to obtain a biaxiallyoriented retardation film. The evaluation results of the characteristicproperties of this film are shown in Table 3.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 4. A frame phenomenon was confirmedwith the eye in the biaxially oriented retardation film. It was foundthat a change in brightness after a durability test was large at thefour corners as shown in Table 4 and therefore a biaxially orientedretardation film of interest could not be obtained.

Comparative Example 4

A polycarbonate copolymer was obtained in the same manner as in Example4 except that monomers shown in Table 3 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 4, itwas stretched to 1.3 times at 156° C. with a longitudinal uniaxialstretching machine and then to 1.7 times at 172° C. with a transverseuniaxial stretching tenter. The draw ratio was then reduced to 1.65times at the last part of the transverse uniaxial stretching machine tocarry out heat setting at 170° C. for 10 seconds so as to obtain abiaxially oriented retardation film. The evaluation results of thecharacteristic properties of this film are shown in Table 3.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 4. A frame phenomenon was confirmedwith the eye in the biaxially oriented retardation film as shown inTable 4. It was found that the difference in brightness between thecenter and the four corners was large even at the beginning and becamelarger after a durability test, and therefore a biaxially orientedretardation film of interest could not be obtained. TABLE 3 Ex. 4 Ex. 5Ex. 6 Ex. 7 C. Ex. 3 C. Ex. 4 Structure of monomer 1 [A] [B] [C] [E] [A][A] (mol % of charged monomer) (50) (55) (53) (50) (50) (100) Structureof monomer 2 [D] [D] [D] [D] [D] (mol % of charged monomer) (50) (45)(47) (50) (50) Heat shrinkage factor (%) 0.07 0.08 0.07 0.08 0.20 0.09Glass transition temperature (° C.) 210 211 230 170 210 156 R(450) (nm)42.4 33.2 21.9 53.6 39.8 43.7 R(550) (nm) 41.3 32.5 21.3 53.1 38.6 40.5R(450)/R(550) 1.03 1.02 1.03 1.01 1.03 1.08 K(550) (nm) 135.9 128.1120.6 227.5 100.5 99.6 Film thickness after stretching (μm) 62 69 65 4063 76 Residue of solvent (%) 0.3 0.5 0.3 0.2 1.2 0.1 Water absorptioncoefficient (wt %) 0.2 0.2 0.2 0.2 0.2 0.2 Intrinsic viscosity (dl/g)0.77 0.91 0.66 0.71 0.77 0.78 Change in R(550) 1.9 1.6 2.1 2.2 3.9 2.9Ex. = Example,C. Ex. = Comparative Example

TABLE 4 Initial evaluation (cd/m²) Evaluation after 500 hours (cd/m²)Visual Four-corner brightness Central brightness Four-corner brightnessCentral brightness evaluation Ex. 4 0.34 0.33 0.36 0.33 OK Ex. 5 0.360.34 0.38 0.34 OK Ex. 6 0.37 0.36 0.39 0.35 OK Ex. 7 0.35 0.33 0.38 0.34OK C. Ex. 3 0.34 0.31 0.49 0.33 NG C. Ex. 4 0.43 0.35 0.68 0.39 NGEx. = Example,C. Ex. = Comparative Example

Example 8

An aqueous solution of sodium hydroxide and ion exchange water were fedto a reactor equipped with a stirrer, thermometer and reflux condenser,monomers [A] and [D] having the above structures were dissolved in thesolution in a molar ratio shown in Table 5, and a small amount ofhydrosulfite was added to the resulting solution. Methylene chloride wasthen added to the solution, and phosgene was blown into it at 20° C. inabout 60 minutes. Further, p-tert-butylphenol was added to emulsify thesolution, and triethylamine was added and stirred at 30° C. for about 3hours to complete a reaction. After the end of the reaction, an organicphase was dispensed, and methylene chloride was evaporated to obtain apolycarbonate copolymer. The composition of the obtained copolymer wasalmost the same as the ratio of the charged monomers shown in Table 6.

This copolymer was dissolved in methylene chloride to prepare a dopehaving a solids content of 18 wt %. A cast film was formed from thisdope to obtain an unstretched film. The residue of the solvent in theunstretched film was 0.9 wt %. Further, this film was stretched to 1.3times at 212° C. with a longitudinal uniaxial stretching machine and to1.42 times at 220° C. with a transverse uniaxial stretching tenter. Thedraw ratio was then reduced to 1.40 times at the last part of thetransverse uniaxial stretching machine to carry out heat setting at 220°C. for 10 seconds so as to obtain a biaxially oriented retardation film.The evaluation results of the characteristic properties of this film areshown in Table 5. The slow axis of this biaxially oriented retardationfilm was existent in a direction (main stretching direction)perpendicular to the flow direction of the transverse uniaxialstretching machine.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 6. It was found that the framephenomenon of the biaxially oriented retardation film was at aninsignificant level as shown in Table 6.

For this frame test, the rolled polarizer film and the rolled biaxiallyoriented retardation film were joined together with an adhesive layer byroll-to-roll in such a manner that the transmission axis (perpendicularto the longitudinal direction) of the polarizer film became parallel tothe slow axis of the biaxially oriented retardation film. When the frametest was also made on this laminate, it was found that the framephenomenon of the laminate was at an insignificant level as well.

This biaxially oriented retardation film was evaluated using thecommercially available VL-151VA liquid crystal monitor making use of avertical alignment mode manufactured by Fujitsu Limited. Thiscommercially available liquid crystal display device comprises tworetardation films and a liquid crystal cell sandwiched between theretardation films. The retardation film on the rear side opposite to theobserver's side of the liquid crystal cell was removed, and the abovebiaxially oriented retardation film was laminated on the liquid crystalcell instead in such a manner that the transmission axis of thepolarizer film and the slow axis of the biaxially oriented retardationfilm became parallel to each other. The lamination angle between thepolarizer film and the liquid crystal cell was made the same as that ofthe commercially available product. Further, the retardation film on thefront side of the liquid crystal cell of the commercially availableproduct was also removed, a retardation film prepared by stretching theabove unstretched film to 1.2 times at 212° C. uniaxially in alongitudinal direction (R(550)=105 nm, K(550)=52 nm) was laminated onthe liquid crystal cell with an adhesive layer therebetween in such amanner that the transmission axis of the polarizer film became parallelto the slow axis of the retardation film. The lamination angle betweenthe polarizer film and the liquid crystal cell was made the same as thatof the commercially available product. When the viewing angle waschecked with the eye, it was found that the viewing angle was wider thanthat of the commercially available product and that a color shift by theviewing angle could be considerably suppressed.

Example 9

A polycarbonate copolymer was obtained in the same manner as in Example8 except that monomers shown in Table 5 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 8, itwas stretched to 1.2 times at 214° C. with a longitudinal uniaxialstretching machine and then to 1.21 times at 221° C. with a transverseuniaxial stretching tenter. The draw ratio was then reduced to 1.2 timesat the last part of the transverse uniaxial stretching machine to carryout heat setting at 221° C. for 10 seconds so as to obtain a biaxiallyoriented retardation film. The evaluation results of the characteristicproperties of this film are shown in Table 5.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 6. It was found that the framephenomenon of the biaxially oriented retardation film was at aninsignificant level as shown in Table 6.

Example 10

A polycarbonate copolymer was obtained in the same manner as in Example8 except that monomers shown in Table 5 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. After a film was formed in the same manner as in Example 8, itwas stretched to 1.2 times at 233° C. with a longitudinal uniaxialstretching machine and then to 1.21 times at 238° C. with a transverseuniaxial stretching tenter. Without reducing the draw ratio at the lastpart of the transverse uniaxial stretching machine, heat setting wascarried out at 240° C. for 10 seconds to obtain a biaxially orientedretardation film. The evaluation results of the characteristicproperties of this film are shown in Table 5.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 6. It was found that the framephenomenon of the biaxially oriented retardation film was at aninsignificant level as shown in Table 6.

Comparative Example 5

A polycarbonate copolymer was obtained in the same manner as in Example8 except that monomers shown in Table 5 were used. The composition ofthe obtained copolymer was almost the same as the ratio of the chargedmonomers. A film was formed by changing drying conditions from those ofExample 8, and the residue of the solvent in the unstretched film wasadjusted to 3 wt %. This film was stretched to 1.2 times at 200° C. witha longitudinal uniaxial stretching machine and then to 1.3 times at 210°C. with a transverse uniaxial stretching tenter to obtain a biaxiallyoriented retardation film. The evaluation results of the characteristicproperties of this film are shown in Table 5.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 6. A frame phenomenon was confirmedwith the eye in the biaxially oriented retardation film as shown inTable 6. It was found that a change in brightness after a durabilitytest was large at the four corners and therefore a biaxially orientedretardation film of interest could not be obtained.

Comparative Example 6

A polycarbonate homopolymer was obtained in the same manner as inExample 8 except that monomers shown in Table 5 were used. Thecomposition of the obtained homopolymer was almost the same as the ratioof the charged monomers. After a film was formed in the same manner asin Example 8, it was stretched to 1.1 times at 156° C. with alongitudinal uniaxial stretching machine and then to 1.13 times at 172°C. with a transverse uniaxial stretching tenter. The draw ratio was thenreduced to 1.11 times at the last part of the transverse uniaxialstretching machine to carry out heat setting at 170° C. for 10 secondsso as to obtain a biaxially oriented retardation film. The evaluationresults of the characteristic properties of this film are shown in Table5.

Further, a frame test was made on this biaxially oriented retardationfilm. The results are shown in Table 6. A frame phenomenon was confirmedwith the eye in the biaxially oriented retardation film as shown inTable 6. It was found that the difference in brightness between thecenter and the four corners was large even at the beginning and becamelarger after a durability test. Therefore, a biaxially orientedretardation film of interest could not be obtained. TABLE 5 Ex. 8 Ex. 9Ex. 10 C. Ex. 5 C. Ex. 6 Structure of monomer 1 [A] [B] [C] [A] [A] (mol% of charged monomer) (50) (55) (53) (50) (100) Structure of monomer 2[D] [D] [D] [D] (mol % of charged monomer) (50) (45) (47) (50) Heatshrinkage factor (%) 0.06 0.08 0.07 0.21 0.09 Glass transitiontemperature (° C.) 210 211 230 210 156 R(450) (nm) 3.19 1.55 6.14 4.3913.29 R(550) (nm) 3.10 1.52 5.96 4.26 12.31 R(450)/R(550) 1.03 1.02 1.031.03 1.08 K(550) (nm) 201.9 95.8 112.6 132.8 135.4 Film thickness afterstretching (μm) 80 71 69 85 79 Residue of solvent (%) 0.3 0.5 0.4 1.20.1 Water absorption coefficient (wt %) 0.2 0.2 0.2 0.2 0.2 Intrinsicviscosity (dl/g) 0.78 0.92 0.65 0.78 0.78 Change in R(550) 0.7 1.3 0.93.7 2.9Ex. = Example,C. Ex. = Comparative Example

TABLE 6 Initial evaluation (cd/m²) Evaluation after 500 hours (cd/m²)Visual Four-corner brightness Central brightness Four-corner brightnessCentral brightness evaluation Ex. 8 0.35 0.33 0.36 0.34 OK Ex. 9 0.370.34 0.38 0.34 OK Ex. 10 0.36 0.35 0.38 0.36 OK C. Ex. 5 0.35 0.31 0.510.32 NG C. Ex. 6 0.46 0.35 0.69 0.38 NGEx. = Example,C. Ex. = Comparative Example

As having been described above, according to the present invention, itis possible to provide a retardation film which has excellent viewingangle characteristics and rarely sees a frame phenomenon in a liquidcrystal display device, particularly a vertical alignment mode liquidcrystal display device, while maintaining the excellent properties of apolycarbonate such as moldability, impact resistance and ruptureresistance, by stretching a polycarbonate having a specific structureuniaxially or biaxially as a polymer to obtain a uniaxially or biaxiallyoriented film having specific physical properties. When this retardationfilm of the present invention is used in a liquid crystal display devicetogether with a polarizer film, there can be provided a liquid crystaldisplay device which rarely experiences display nonuniformity and solvesa frame problem.

1. A uni- or bi-axially oriented film (A) which comprises a polymer orpolymer mixture containing a recurring unit represented by the followingformula (I):

wherein R¹ to R⁸ are each independently a member selected from the groupconsisting of hydrogen atom, halogen atom, hydrocarbon group having 1 to6 carbon atoms and hydrocarbon-O-group having 1 to 6 carbon atoms, and Xis represented by the following formula (i)-1:

wherein R³⁰ and R³¹ are each independently a halogen atom or alkyl grouphaving 1 to 3 carbon atoms, and n and m are each independently aninteger of 0 to 4, each of the polymer and the polymer mixturecontaining the recurring unit represented by the above formula (I) in anamount of 30 to 60 mol % based on the total of all the recurring unitsof the polymer or polymer mixture and having a glass transitiontemperature of 165° C. or higher, (B) which has a heat shrinkage factorwhen it is heated at 90° C. for 500 hours under no load of 0.1% or less,and (C) which satisfies the following formula (1):1≦R(450)/R(550)≦1.06  (1) wherein R(450) and R(550) are retardationswithin the film plane at wavelengths of 450 nm and 550 nm, respectively.2. The film of claim 1, wherein the amount of the recurring unitrepresented by the formula (I) is larger than 30 mol % based on thetotal of all the recurring units.
 3. The film of claim 1 which is auniaxially oriented film further satisfying the following expressions(2) and (3) at the same time:R(550)>K(550)  (2)R(550)>20 nm  (3) wherein R(550) is as defined in the above expression(1) and K(550) is a value (nm) defined by the following expression (4)at a wavelength of 550 nm:K=[(n _(x) +n _(y))/2−n _(x) ]×d  (4) wherein n_(x), n_(y) and n_(z) arerefractive indices in x axis, y axis and z axis directions of the film,respectively, and d is the thickness (nm) of the film.
 4. The film ofclaim 1 which is a biaxially oriented film further satisfying thefollowing expression (2′) and the above expression (3) at the same time:R(550)≦K(550)  (2′) wherein R(550) and K(550) are as defined in theabove expressions.
 5. The film of claim 4 which satisfies the followingexpression (1′):1≦R(450)/R(550)≦1.05  (1′) wherein R(450) and R(550) are as defined inthe above expressions.
 6. The film of claim 1 which is a biaxiallyoriented film further satisfying the above expression (2′), thefollowing expression (3′) and the following expression (5) at the sametime:R(550)≦20 nm  (3′)K(550)≧50 nm  (5) wherein R(550) and K(550) are as defined in the aboveexpressions.
 7. The film of claim 1, wherein the polymer or polymermixture has a glass transition temperature of 200° C. or higher.
 8. Thefilm of claim 1 which is a retardation film.
 9. A laminated polarizerfilm which comprises the film of any one of claims 1 to 6 and apolarizer film in a laminated form.
 10. The laminated polarizer film ofclaim 9 which is laminated in such a manner that the transmission axisof the polarizer film becomes parallel to the slow axis within the planeof the film.
 11. A liquid crystal display device comprising thelaminated polarizer film of claim 9 or
 10. 12. The liquid crystaldisplay device of claim 11 which is in a vertical alignment mode.